The internal tubular element is a structure made of copper, or of copper alloy, manufactured from a block or constituted by four assembled plates defining a passage for the cast product and which is energetically cooled, generally by circulation of water, in order to cause peripheral solidification of the cast metal.
One of the major problems encountered in the continuous casting of metals, especially of steel, whether this casting is of the vertical, curved or horizontal type, is that the solidified shell resulting from peripheral cooling of the cast bar tends to catch on the internal surface of the tubular element. Because of this catching, fissures may be produced in the solidified crust as the bar progresses in the mold, such fissures being the cause of breakouts and surface defects in the products which are obtained.
It is known, e.g., from Belgian Patent No. 886,924, French Patent No. 2,471,821 and Japanese Patent No. 86432/79, to reduce friction, and thus to promote lubrication between the cast metal and the inner mold wall, by applying to the latter, by means of transducers attached laterally to the internal tubular element, ultrasound vibrations in a direction perpendicular to the surface of the element, i.e., perpendicular to the casting axis. In this manner, the danger of breakouts could be limited and the surface quality of the cast products improved.
However, this known technique has certain important disadvantages. Under the effect of such vibrations, the wall of the tubular element is subjected to deformations toward the mold interior, deformations whose amplitude reaches a maximum only in the vicinity of the location where the ultrasonic transducer is applied against the said wall. The result is a limited overall reduction in the frictional forces, generally of the order of 50%, unless several transducers are provided, as proposed in the French patent.
Moreover, the very large mass of liquid steel present in the mold tends, through its inertia, to resist the vibrations transmitted at the wall. In order to mitigate this effect, it is necessary to reinforce the latter and, consequently, to make use of heavy and onerous molds and too complex structures because of the internal channels required for cooling this type of mold.
Also known, from British Patent No. 2,108,878, is a technique for bringing into longitudinal oscillation a continuous casting mold supported for this purpose on oscillators attached to the frame of a casting machine. In this technique, it is thus the entire mold assembly which oscillates, not only the internal tubular element, so that it would not be possible to place into oscillation at ultrasonic frequency such a mass of several tons, not to mention more than ten tons in the case of casting products of large cross section, such as slabs.